The present invention relates in general to telecommunication techniques. More particularly, the invention provides a method and system for chirped light source. Merely by way of example, the invention is described as it applies to optical networks, but it should be recognized that the invention has a broader range of applicability.
In optical transmission networks, signals are generally driven by light sources. Often signals in an optical transmission network travel over hundreds or even thousands of miles. An important characteristics of optical fiber transmission is fiber nonlinearity that can cause, among other things, dispersion penalty and thus limit the transmission distance. For example, nonlinearity may create self-phase modulation (SPM) effect. To improve the tolerance for nonlinear impairment of light sources, a system for transmitting signals with spectrally enriched optical pulses may be used. For example, a Directly Modulated Lasers (DML) source may be used to produce spectrally enriched optical pulses with simultaneous amplitude and frequency modulation. Spectrally enriched optical pulses are usually more tolerant to fiber nonlinearity. For example, a DML can, among other things, generate a positive chirp to counter-balance the (SPM) effect and thus the positively chirped pulse is more tolerant to fiber nonlinearity.
FIG. 1 is a simplified diagram of a conventional system for producing spectrally enriched optical pulses. Spectrally enriched optical pulses may be in form of chirped light. For example, chirped light is frequency-modulated light. As another example, chirped light has a frequency that varies with time, the variation of frequency staying within a range. The system 100 includes a DML 110, a driving signal source 120, an RF amplifier 130, and a voltage source 140. The system 100 produces chirped light output 150. The driving signal source 120 provides driving signals to the RF amplifier 130. Driving signals produced by the driving signal source 120 may be an NRZ binary signal associated with a predetermined minimum pulse duration. Alternatively, driving signals may be alternating current (AC) or clock signals. After the RF amplifier 130 receives driving signals from the driving signal source 120, the RF amplifier 130 amplifies the driving signals with a predetermined amplification factor. After amplification, the RF amplifier 130 transmits the driving signals to the DML 110. The DML 110 first couples the amplified driving signals with a bias voltage supplied by the voltage source 140. As an example, a bias voltage is a direct current (DC) voltage. Then the amplified driving signals modulate the DML 110. As an example, the DML output 150 is an optical signal that has been modulated in amplitude. Sometimes, the optical signal is also frequency-modulated.
Generally, the wavelength of DML sources may be tuned. One of the methods for tuning is varying the device temperature. However the spectral tuning range is often limited to about 3 nm, which covers at most 3 optical channels in case of 100 GHz dense wavelength division multiplexing (DWDM) channel spacing. A modern DWDM optical transport system often needs to use several tens of channels. The narrow spectral tuning range of conventional DML sources could mean that more than 10 DMLs are needed to cover typical operating spectral range of greater than 30 nm in a DWDM optical system. Utilizing multiple, often in an order of tens, of DMLs leads to high inventory cost and limits the flexibility in applications where dynamical wavelength provisioning is required.
Hence it is highly desirable to improve techniques for generating chirped light source that offers both wide spectral tunable range in wavelength and substantially the same chirp characteristics of the conventional DMLs.